1. Download 3.1.2 custom firmware from here (this version is tested and working 100% in windows)
2. Make sure your 3.0 or 3.0.1 Iphone 3GS is jailbroken..
you may use blackra1n to do so or already have done it with redsn0w.
3. Put your Iphone in Recovery Mode (the itune and cable showing on screen) (**NOT DFU** , In DFU mode you will get error 1600 )
and just restore to this holding your SHIFT button as usal to pick custom restore file.
4. Run ULTRASN0W*
http://www.megaupload.com/?d=WQR1IZZ9